Sulfuric acid is the largest volume inorganic acid currently in use and is generally considered to be the most important industrial chemical. Cold diluted sulfuric acid may be readily handled in most situations. On the other hand, the production and handling of hot concentrated sulfuric acid presents rather specialized problems in the field of corrosion.
The hallmark of alloys based upon the addition of large amounts of chromium to iron, nickel or some combination of iron and nickel is the remarkable resistance of the resultant alloys to oxidizing chemical substances when oxygen is present in some available form.
In modern usage an agent that will cause metallic atoms to lose electrons is called an oxidizing agent. The loss of electrons by atoms is considered to be oxidation, so that metals and alloys may be "oxidized" by substances that contain no oxygen at all because they remove electrons from the metals.
It is the property of the chromium-bearing alloys in the presence of available oxygen to somehow readily and quickly form a protective or passive surface against further attack. If this passive surface is disturbed or disrupted under these conditions it will quickly repair itself or repassivate. In the absence of oxygen these alloys are ordinarily not able to become passive or to regain passivity on a disrupted surface.
Hot concentrated sulfuric acid does not ordinarily contain sufficient dissolved oxygen to passivate alloys of chromium which contain various proportions of nickel and iron. However, it has been learned over the years that higher proportions of nickel in such alloys are beneficial in establishing passivity in hot concentrated sulfuric acid. Also, copper, molybdenum and silicon are additional elements that tend to passivate chromium-bearing alloys in hot concentrated sulfuric acid, so that the resultant alloys, with suitable proportions of these various elements, may be quite resistant to this very corrosive substance.
The presence of 14% to 17% silicon and a few percent of molybdenum or copper in iron result in alloys that are quite resistant to hot concentrated sulfuric acid and low in strategic element content. However, because silicon is a non-metallic element, these high-silicon alloys are even more brittle than glass and hence of very limited application. There is a parallel situation when about 8% to 10% silicon and a few percent of copper are added to nickel. This alloy is a bit less brittle, quite resistant to the acid, much higher in cost, and again, of very limited application.
Glasses are also generally quite resistant to sulfuric acid but ordinarily limited in their use to the packaging and transportation of fairly small quantities of cold acid of the order of a gallon or a liter. This is due to the pronounced susceptibility of glasses to cracking or shattering by either a structural strain or blow or by a sudden change of temperature.
Lead is also resistant but is limited in employment as a sheathing or lining material because lead has extremely low structural strength.
Also, a few precious elements, such as platinum, are quite resistant to hot concentrated sulfuric acid but are so scarce that their extensive use in commercial acid production and handling is quite out of the question.
Furthermore, some industrial streams of hot concentrated sulfuric acid solutions may contain sludges or particulate matter and/or free air or vapor bubbles. Either of these conditions may lead to severe erosion problems in addition to chemical attack. It is for this reason that many alloys developed for the handling of hot concentrated sulfuric acid are also relatively hard even at the expense of giving up toughness, ductility, fabricability and weldability. However, those properties are usually so important that a compromise is almost always desirable.
Samuel Parr disclosed in 1914, in U.S. Pat. No. 1,115,239, an alloy of about 63% nickel, 20% chromium, 5% molybdneum, 5% copper and 2% tungsten, in which small quantities of iron, silicon, mangenese, titanium, boron and aluminum may also be present. This alloy had useful resistance to hot sulfuric acid as well as to several other acids and chemical substances and could be easily air melted, cast, forged and drawn.
It wasn't until 1937 that LaBour, U.S. Pat. No. 2,103,855, revealed a similar alloy which typically contained the major elements of Parr Plus about 4% silicon and up to about 8% iron. The alloy of LaBour had relatively good resistance to the corrosion of hot solutions of many substances including sulfuric acid but was hampered by relatively high carbon contents of about 0.2 to 0.3%. LaBour also represented the first reported such alloy to give up toughness for hardness.
Then in 1952, Jackson, in U.S. Pat. No. 2,597,495, disclosed an alloy intended for improved fabricability. The alloy of Jackson was, in some respects, a combination of the alloys of Parr and LaBour but of lower carbon and copper contents with the elimination of tungsten. However, Jackson's alloys were of even lower resistance to hot concentrated sulfuric acid.
The mechanical properties of alloys-pendulum then swung back in the hard-brittle direction with the issuance in 1960 of patents to Johnson, U.S. Pat. No. 2,938,786, and to Boyd, Langton and Johnson, U.S. Pat. No. 2,938,787. Both patents provided for silicon contents up to about 6% or 7% plus additions of boron up to about 0.55%. Jackson '786 allowed slightly higher iron additions and essentially covered chromium levels below 26%, while the '787 patent covered chromium levels from 26% to 30% and permitted an iron content to only 3.5%.
For the higher-silicon variations of both of these alloys, resistance to concentrated sulfuric acid up to 100.degree. C. is quite good. The corrosion resistance of both alloys to hot concentrated acid deteriorates rapidly, however, when the silicon content drops much below about 5%. And, as is the usual case, brittleness and extreme lack of fabricability, workability and weldability remain as characteristics of the alloys with the higher-silicon contents that are so resistant to the corrosive effects of the hot acid. Also, as with the other alloys described above, these alloys are characterized by having to be formulated from relatively pure raw materials due to their low permissible iron levels.
In both U.S. Pat. Nos. 2,938,786 and 2,938,787 it is stated that the addition of the non-metallic element boron when added with the non-metallic element silicon in certain prescribed proportions, actually improves mechanical properties without sacrificing corrosion resistance or hardness. Nonetheless, the alloys of those patents are quite brittle even though they have excellent corrosion resistance to hot concentrated sulfuric acid, especially when silicon contents approach the 6% to 6.5% levels. In industrial applications these alloys are usually furnished at the 3.5% silicon level with some sacrifice in corrosion resistance in order to gain at least some reduction in brittleness. But, since the iron content has to be held to very low proportions, the resultant alloys have to be formed from relatively pure sources of chromium, molybdenum, silicon and nickel. Nickel and silicon are ordinarily available in the pure or concentrated form, but chromium and molybdenum are much more costly and difficult to employ in air melting practice as pure elements than they are when usable as ferro-alloys.
Boyd, Langton and Johnson also disclosed a third sulfuric acid-resistant alloy in U.S. Pat. No. 3,008,822, in 1961, which was designed to provide sufficient fabricability to afford rolled or wrought forms. This was essentially a low-silicon, boron-free version of the alloy of U.S. Pat. No. 2,938,787. The alloy was fairly tough and fabricable but not nearly as resistant to hot concentrated sulfuric acid as are the high-silicon versions. Also, only 1.5% iron or less can be tolerated, requiring that the alloy be formulated from high-purity forms of the constituent elements.
Still later, in 1973, Johnson, in U.S. Pat. No. 3,758,296, discloses an alloy of higher chromium content along with somewhat lower molybdenum and copper levels. This alloy was stated to be able to tolerate high iron contents, permitting the use of ferro-alloys in place of pure chromium and molybdenum. The alloy also provided for somewhat reduced nickel contents but in so doing employed relatively high manganese contents plus the inclusion of the scarce and expensive element cobalt. This alloy retained the amount of boron at levels reduced from prior patents along with silicon contents of 4% or less. The alloy is said to have good resistance to hot concentrated sulfuric acid when its constituent elements are present in optimum proportions, but it is so brittle that it is extremely difficult to cast without cracking and does not possess weldability by any ordinary methods.
However, the commercial alloys of Johnson (U.S. Pat. No. 3,758,296) are relatively unstable in metallurgical structure. They show some slight tensile elongation if cast into rather small castings or thin sections, but display extreme brittleness when cast into heavier sections. That patent provides for nickel contents to 48%, but the alloys then require iron contents of 3% or less. Use of ferro-alloys is no longer possible.
But aside from that, the highest nickel content Johnson alloys are still hard and brittle and suffer drastic loss of resistance to hot concentrated sulfuric acid corrosion. The nickel-equivalency of the Johnson alloys range from about 33% to 43.7%, excluding manganese and copper, whose effects are minor but including the estimated effects of carbon. The commercially employed Johnson alloy has about 40% to 41% nickel-equivalency based upon the same constituent elements. The commercial alloy has a chromium equivalency of about 49%. Metallugically this would require about 50% or higher nickel-equivalency to maintain a fairly stable austenitic matrix. Since the Johnson alloys do not meet this balance, they tend to have very unstable matrices resulting in very hard and brittle castings unless they are produced in only very thin cross sections or are solution heat treated for about four hours at the relatively high temperature of about 2050.degree. F. This is near the incipient fusion temperature of about 2100.degree. F. and is, therefore, a difficult and costly heat treatment which results in irreversible casting damage if the heat-treating furnace controls are slightly out of calibration.
From the foregoing it is evident that a commercially useful alloy of high enough ductility to be fabricable in sheets, tubes, etc., and still resistant to very hot concentrated sulfuric acid would be most desirable. The problem has been, however, that such an alloy has not been forthcoming because of the apparent need for large proportions of chromium, molybdenum, and silicon, combined with copper as well as high nickel levels, and, usually, low levels of iron. Nevertheless, it is still desirable to have a castable alloy of at least modest tensile elongation that has good resistance to hot concentrated sulfuric acid and still capable of being formulated with ferro-alloys and ordinary air-melting equipment. It is also desirable that such an alloy may be hardenable after machining in order to better resist the erosion encountered in services that involve contact with particulate matter or vapor bubbles.
Thus the prior art alloys for handling hot concentrated sulfuric acid have been encumbered with the same problem. The chromium, molybdenum and silicon levels required for corrosion purposes have simply tended to be too high to be structurally offset by nickel even when nickel is at the highest level possible.